The present invention relates to airborne Tacan equipment and more particularly to improved airborne Tacan equipment.
The concept and operational characteristics of Tacan systems are well known. A detailed description may be found in MIL STD 291B. Basically, the Tacan system allows an aircraft to determine its position relative to a Tacan ground beacon. Basically, Tacan is a radio aerial navigation system of the polar-coordinate type wherein there is a bearing facility that provides on the aircraft a meter indication of its direction in degrees of bearing from a ground beacon selected by the pilot. Also, there is a distance facility that provides on the aircraft a meter indication in nautical miles of its distance from the selected ground beacon. Knowing the bearing and distance from a specific geographic point (the selected ground beacon), the pilot can fix his position on a chart.
The bearing information is extracted from phase measurements, and, therefore, it is necessary to have a fixed frequency of the same frequency that is received in all directions from the beacon with identical phase. As a convenient standard, as defined in MIL STD 291B, the phase of the reference wave is adjusted so that it is transmitted at the same instant of time that the maximum of the rotating cardioid lobe pattern aims east. In any case, the airborne electrical phase difference measurement would numerically represent the aircraft's geographic bearing, which is conventionally measured clockwise from north. Actually, any other standard adjustment for the reference wave might be adopted. The airborne phase measuring circuits would simply be zero-calibrated accordingly. The north reference group of pulses furnishes suitable reference for measuring the phase of the 15 cycle component of the envelope wave.
To furnish a suitable reference for measuring the phase of a 135 cycle component of the envelope wave, the ground equipment transmits appropriately timed encoded reference signals similar to the north reference group of signals. These secondary reference signals are referred to as auxiliary reference pulse groups. In one rotation of the beacon antenna, eight of these groups are transmitted, separated by 40.degree. of rotation. The ninth position is occupied by the north reference signal group.
In airborne Tacan equipment, amplifiers are used to bring the amplitude of the Tacan pulses to a level sufficient for extraction of bearing information and must be such as to (1) preserve amplitude modulation of the pulse train or modulation components which carry the bearing information; and (2) accept input signal variations of large amplitudes of approximately 90 dB (decibel).
These two requirements are usually satisfied by a linear amplitude of limited linear range of approximately 12 to 15 dB with automatic gain control (AGC).
There are two major drawbacks to the utilization of this type of amplifier. The AGC controlled loop adds complexity to the amplifier and the time constants of the AGC loop are a compromise. The time constants must be chosen to preserve the 15 Hz (Hertz) modulation component of the received signal and to respond to rapid signal variations.